Sulfonic-acid-derived-group-containing fluoropolymers obtained by copolymerizing tetrafluoroethylene and a —SO2F-containing perfluorovinyl ether are known to be useful, in the form resulting from hydrolysis of —SO2F, as electrolyte membranes in fuel cells, chemical sensors and so forth.
The hydrolyzates of those sulfonic-acid-derived-group-containing fluoropolymers, when used, for example, as fuel cell electrolyte membranes for a long period of time, reportedly produce, as a result of deterioration thereof, such a problem as contamination of the wastewater discharged from the fuel cells with HF.
Reportedly, an improvement can be produced in this respect by a certain kind of stabilization treatment, namely when such sulfonic-acid-derived-group-containing fluoropolymers in solid state are brought into contact with a fluorine atom radical-generating compound, such as gaseous fluorine, at 20 to 300° C. to thereby convert at least 40% of unstable groups at polymer chain termini to stable groups (cf. e.g. Patent Document 1: Japanese Patent Publication S46-23245).
However, such prior art stabilization treatment has a problem in that in particular when the sulfonic-acid-derived-group-containing fluoropolymer to be treated is one obtained by emulsion polymerization, the rate of conversion of unstable groups to stable groups becomes insufficient and, therefore, discoloration and frothing, among others, occur in the step of melt molding.